(1) Field of the Invention
The present invention relates to a dual frequency primary radiator, such as a parabolic antenna etc., which can handle two frequencies with two components of polarization.
(2) Description of the Prior Art
As a parabolic antenna which can handle two different frequencies, a type as shown in FIGS. 1 and 2 has been conventionally known. FIG. 1 is an overall view of the parabolic antenna, and FIG. 2 is an enlarged view of a dual frequency primary radiator. In the following description, the lower frequency band of two different frequencies will be referred to as f.sub.L, and the higher frequency band will be referred to as f.sub.H.
The antenna shown in FIG. 1 has a dual frequency primary radiator 110, positioned at the focal point of a parabolic reflector 100. This primary radiator 110 is composed of, as shown in FIG. 2, an f.sub.L primary radiator 101 and an f.sub.H primary radiator 102 with waveguides 103 and 104. Waveguides 103 and 104 have feedhorns 111 and 112, respectively at their one end, forming cone-shaped openings, and have plate-like reflecting means 107 and 108 enclosing the other end thereof. The f.sub.H waveguide 104 is arranged concentrically inside the f.sub.L waveguide 103. An f.sub.L coaxial/waveguide conversion feeder 105 is provided for f.sub.L waveguide 103 and an f.sub.H coaxial/waveguide conversion feeder 106 is provided for f.sub.H waveguide 104.
Referring to FIG. 2, consider a case where a radiowave is transmitted from the antenna. An f.sub.H signal from a transmitter is fed to waveguide 104 from feeder 106 via a coaxial cable line so that the signal is radiated into space from primary radiator 102, which is in turn is reflected by the parabolic reflector and then transmitted. On the other hand, a received f.sub.L signal is input to a primary radiator 101 through the parabolic reflector and then the signal, passing through waveguide 103 and feeder 105, enters the receiver, where the received signal can be picked up.
Waveguide 103 allowing the passage of the f.sub.L signal serves as the outer conductor of the coaxial waveguide, and waveguide 104 allowing the passage of the f.sub.H signal functions as the central conductor for waveguide 103. Concerning coaxial-waveguide conversion feeders, in the case where f.sub.L and f.sub.H frequencies are of single polarization, f.sub.L and f.sub.H feeders 105 and 106 are provided one for each frequency and positioned 90.degree. apart from each other in order not to interfere with each other.
When each of frequencies f.sub.L and f.sub.H is of two types of polarization (i.e., horizontal/vertical polarization for linearly polarized waves, right-hand and left-hand circular polarization for circularly polarized waves), two coaxial-waveguide conversion feeders for each of frequencies f.sub.L and f.sub.H need to be provided in an orthogonal manner as shown in FIG. 3. More specifically, when the transmission or received signal is of a linearly polarized wave, f.sub.L feeder 105V for vertical polarization, f.sub.L feeder 105H for horizontal polarization and f.sub.H feeder 106V for vertical polarization and f.sub.H feeder 106H for horizontal polarization are needed. Concerning f.sub.H feeders 106V and 106H, in order to set the characteristic impedance of the feeder at 50 .OMEGA., in portions other than f.sub.H waveguide 104, they need to have a coaxial cable configuration.
This coaxial cable configuration is composed of a center conductor 151, an insulator 152 and an outer conductor 153 coaxially arranged in this order as shown in FIG. 4, and the characteristic impedance will be determined depending upon the outside diameter of the center conductor, the inside diameter of the outer conductor and the dielectric constant of the insulating material.
A feeder for transforming a coaxial line into a waveguide needs a reflecting means (designated at 107 and 108, as shown in FIG. 3) which is disposed at a distance therefrom of about one quarter of a guide wavelength (.lambda.g/4) in the direction opposite the signal propagating direction. The reflecting means 107 and 108 are plates for enclosing the waveguides, as shown in FIG. 3. It is also possible to provide a bar-shaped reflecting means 109, as shown in FIG. 5, which is arranged in parallel with the electric field component of the signal. The reflecting means need be formed of a conductive material so as to provide electric connection with the waveguide.
In the above case, two coaxial/waveguide conversion feeders arranged orthogonally are needed. In this configuration shown in FIG. 3, for signal transmission, an f.sub.L signal from f.sub.L feeder 105V will be reflected by the outer conductor of f.sub.H feeder 106V, and an f.sub.L signal from f.sub.L feeder 105H is reflected by the outer conductor of f.sub.L feeder 106H, so that the two f.sub.L signals cannot reach feedhorn 111. For signal reception, an f.sub.L signal from feedhorn 111 will be reflected by the outer conductors of f.sub.H feeders 106V and 106H so that the signal cannot reach either f.sub.L feeders 105V or 105H. This situation will be also the same in the case where f.sub.H feeder 106V is provided opposite f.sub.L feeder 105V (180.degree. apart) as shown in FIG. 6.